1. Field of the Invention
The invention relates to a device for rapidly, and with low noise, slaving the frequency and phase of a signal to that of an imposed signal, of a type known as phase locked loop. The device finds applications particularly in the construction of pilot oscillators in FM radio transmitters or else for recovering a clock signal timing the transmission of digitized data. It may for example be used in seismic data transmission systems.
2. Description of the Prior Art
As will be seen further on in greater detail, in connection with FIG. 1, a conventional phase locked loop frequency f.sub.R is used as reference.
The signal from the oscillator is applied for example to an element which divides its frequency by a fixed or adjustable factor M. The resultant signal is mixed by a modulator with another signal whose frequency is a sub multiple (in a given ratio N) of that of a signal from an adjustable frequency oscillator such as a voltage controlled oscillator (VCO). The electric voltage from the modulator, reduced to its low frequency component by a filter of the first or second order, is applied to the control input of the oscillator (VCO). Such a loop has a property of locking the frequency of the signal from the oscillator (VCO) at a frequency f.sub.s proportional to the frequency f.sub.R of the reference oscillator, the ratio of proportionality being equal to N/M. The adjustment or locking time of the loop varies, as is known, proportionally to the time constant of the integrating filter and in inverse ratio to the gain of the loop characterized more particularly by a factor K.sub.vco. This factor designates the ratio between the frequency variation of the controlled oscillator (VCO) and the control voltage causing this variation and is expressed in kilohertz or megahertz per volt (kHz/V or MHz/V).
Such a locked loop is also characterized by its phase "noise", this term designating the fluctuations of the phase of the signal at the output of the controlled oscillator, after setting of the loop. The phase noise measured is generally proportional to the ratio between the frequency f.sub.s of the signal from the controlled oscillator and that of the reference signal and varies in inverse ratio to the response time of the filter disposed in the feed back (or servo) loop. Since the locking speed of the loop depends directly on this response time, it can be seen that it is difficult to conciliate high locking speed and low phase noise.
In the patent JP 60 249 429 a phase locked loop is described in which the frequency emitted by an oscillator of the VCO type is adjusted by a servo circuit comprising two low pass filters whose time constants are different from each other. A switch controlled by selection means makes it possible to connect successively the fastest then the slowest after re-initialization when frequency tuning is almost achieved.
The device of the invention makes it possible to make the frequency (and the phase) of a signal dependent on that of an imposed signal coming from a reference oscillator, with both a very short response time and a phase noise held at a very low level. It comprises a controlled oscillator whose oscillation frequency can be varied by applying a control voltage and a feed-back (or servo) loop for bringing the control voltage to an optimum value capable of stabilizing the slaved oscillator frequency, the phase noise and the response speed inherent to this loop being relatively low.